1. Field of the Invention
This invention relates to seismic exploration systems and particularly to an improved transceiver for use in a data telemetric system.
2. Description of the Related Art
Recent developments in seismic exploration demand the use of several hundred to thousand seismic sensor groups, each group containing one or more sensors electrically interconnected to form a single channel. Early telemetric systems required cables having many hundred conductor pairs, one pair for each sensor group, to transmit analog seismic-data signals from the sensor group to a central, multichannel, data processing and recording unit. Cables containing individual sensor conductor pairs become impractical for use in large spreads because of the sheer bulk of the cable. Conventional telemetric systems have much fewer conductor pairs for transmitting data, interrogation, and control signals to the entire spread.
U.S. Pat. Nos. 3,881,166 and 3,883,725 issued to Fort et al. disclose a telemetric system consisting essentially of a plurality of cable sections interconnected by a like number of array terminals spaced over the ground in a two-dimensional array. Each array terminal has a plurality of conductor pairs, each pair connected to a sensor or sensor group. Additionally each array terminal contains a plurality of input-output ports for receiving a similar number of cable sections. Each cable section includes conductors for transmitting power, data, and control signals.
The array terminals have a logic network inside, arranged so the control signals will be received by the first terminal, processed, and be retransmitted to a second array terminal through the cable section connected to the next sequential port. Data transmitted back to the recording unit indicates which port is in use and which port is connected to the recording unit.
U.S. Pat. No. 4,117,448 assigned to the assignee of this invention consists of a plurality of cable sections laid end-to-end along a predetermined line of survey. The cable sections are interconnected by a like number of transceiver modules. Each cable section contains a plurality of conductor pairs and a bidirectional telemetric link. The conductor pairs transmit power and connect the sensor groups to the transceiver modules. The telemetric link transmits control and data signals between each transceiver module and the recording unit.
The transceiver modules contain well-known signal multiplexing and digitizing logic to accept analog input signals from the sensor groups that are interconnected externally to the cable sections. Signals from the sensor groups associated with a given cable section and transceiver module are sampled in sequence, digitized as digital data, and transmitted by the transceiver module over the telemetric link to an adjacent transceiver module. The adjacent transceiver module receives and retransmits the data to another transceiver and so on until the data are received by the recording and control unit. According to a preselected transmitting sequence, the addresses of the transceivers from which the data arrive at the recording and control unit are known.
In the above-identified patents, the transceiver modules and cable sections are unpolarized. The systems can receive data interrogation signals from a plurality of directions and are capable of determining which direction to transmit the data to the recording and control unit. Each transceiver module contains an electrical direction sensing device which determines the direction in which the recording and control unit is located. The direction sensing signal is maintained within each transceiver module until electrically reset by the recording and control unit.
A major disadvantage to the aforementioned patents is that all the transceivers or terminals are interconnected in series to the recording and control unit. These systems are subject to a series reliability law. That is, each transceiver module is dependent on the previous transceiver module for the retransmission of signals. If a transceiver module becomes inoperational or malfunctions, the following transceiver modules become crippled or reflect the malfunction in the bad transceiver.
A second disadvantage of the above art is related to the series reliability law. That is, the greater number of times a signal is retransmitted, the greater the chance for the signal to be distorted.
It is an object of the instant invention to provide a telemetric system where at least one continuous broadband telemetric channel interconnects a plurality of transceiver modules thereby reducing the number of signal retransmissions.
It is another object of this invention to provide a telemetric system where adjacent transceiver modules are mutually independent of each other.
It is yet another object of this invention to provide a telemetric system where the data are not influenced by electromagnetic interference.
It is yet a further object of this invention to provide a telemetric system where the data are repeated only when necessary to maintain a prescribed signal strength.